whew! 500watts is more than enough for you. If you swap out all of your lighting with LED's you will find that 150watts will be more than enough for this build, even if you run a 65watt halogen headlight. Unless you see yourself upgrading and building a more powerful system down the road...
Keep your eye open for deals on these: ww.vicorpower.com/products?productType=cfg&productKey=V72B15C250BL

They are super small and reliable. Very powerful for the space they use up. Hard to find a good deal on these, but they do come upon ebay occasionally. You just need to be aware of the model #'s as they make a huge variety of different inputs/outputs and wattages.

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Vicor are good (I used them all the time for work and for my first Lead acid BMS and for my main DC-DC converter, but I had trouble finding them in this voltage range, at this power level, for under $200. Used is good, but who knows the internal stress/condition. The pins are Tinned (even if you get the batmod), and corrode eventually. Just keep those things in mind.